The laser diode of the present invention is mounted on the heat sink made of insulating material such as aluminum nitride (AlN). On the heat sink, a metal film, evaporated gold film, is provided and the laser diode is mounted on the heat sink such that the anode electrode of the laser diode faces and is in contact with the metal film. The heat sink is mounted on the grounded metal member, thus the metal member, the heat sink and the metal film forms an capacitor. The bias for the laser diode is provided via the metal film, thereby stabilizing the bias supply even when the operational frequency for the laser diode is over 10 Gbps.